This research has two basic objectives: (1) The cultivation of pathogenic Treponema pallidum in tissue culture. We have demonstrated that virulent T. pallidum can survive in gradient cultures of Sf1Ep cells for at least 21 days. We have also observed a 3- to 5-fold increase in numbers of T. pallidum during this period. In contrast, T. pertenue attached poorly to tissue culture cells, and no apparent increase in numbers was seen. We are continuing to investigate the phenomenon of increased attachments and are attempting to subculture the treponemes in vitro. (2) Studies on the genetics of T. pallidum. We are utilizing treponemal DNAs labeled with 125I in homology assays to study the genetic relationship between strains of T. pallidum and other pathogenic noncultivable treponemes, as well as between these and nonpathogenic cultivable treponemes.